Inside Kyuzo Gym, in an industrial estate in Dublin's Glasnevin, a booming reverberation overhead ebbs and flows so that it sounds like rolling thunder getting ever louder as it approaches.

The sound comes from the feet of dozens of children, some as young as five, running from one end of a 2,000sq ft canvas to the other, excitedly waiting for their instructor to get a mixed martial arts (MMA) class under way.

When it does, the children pair off, and carefully follow instructions. Despite their miniature frames, they adeptly grapple each other; trying to force opponents to the ground. No striking is allowed.

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At the end of each exercise, the children line up to hear instructions for the next. For the most part, they listen attentively. One particular manoeuvre, the coach says, “may look cool, but can leave you exposed”. Some, in return, nod gravely.

Once the class ends, they line up again. Everybody shakes hands before they dart off in different directions. Barry Oglesby (38), who owns the gym in Ballyboggan Business Park, says MMA fosters a sense of mutual respect between the children.

"Pretty much all the kids that come here are from the Finglas, Cabra, Glasnevin areas," he says. "We've got adults training here who have been with us since they were kids, so they've been with us 10 years.

“There’s a lot that the sport can teach: discipline, having them work towards targets, whether it’s physical improvements or trying to get a new belt, or even something as simple as learning one new skill.”

The popularity of MMA has grown exponentially in the Republic since the loud and unapologetically brash Conor McGregor declared to a global television audience of millions that the Irish had not come to take part but – instead – to take over.

“MMA seems so big now and people are coming in because it’s popular,” says Oglesby. “We have a national hero now. During Italia ’90, we were all playing football on the road. It’s the same type of hysteria. This is just the way it is right now.”

Call for a ban

MMA has come in for heavy criticism since Portuguese fighter Joao Carvalho died at Beaumont Hospital in April after a Total Extreme Fighting bout against Irish fighter Charlie Ward at the National Stadium.

In the days afterwards, some neurologists called for the sport to be banned. Weeks before Carvalho died, former athlete Sonia O’Sullivan had suggested that MMA had “entered the death zone”.

So why are parents encouraging their children to get involved? “I’ve literally never had a parent walk in here and say they want their child to fight,” says Oglesby.

“They’re bringing them down looking for an activity, looking for something healthy to get them out the door. They want them to expend some energy, learn to be disciplined, get off the Xbox.

“Every now and again you hear of a parent saying the child is getting picked on in school. It’s rare, but it does happen. What you don’t want is your child to go over, take a kid down in a playground, and hit them. That’s not what we do. It’s the same with the adults. Fighters tend not to get into fights outside the ring or the cage. They don’t feel there’s that necessity. If you’re good at something like this, you have a deep-rooted confidence that tells you that you don’t have to react that way.

“We start with kids as young as four. At that age, they’re learning about gymnastic movements, balance, cart-wheeling, hop-scotching. We teach them how to utilise their bodies in different ways.

“We also teach them a bit of technique. We might teach them a pin, how to hold an opponent down, or how to do a trip. We try to get them into the idea of having a healthy lifestyle and being active. We don’t coach striking until they’re in their teens.”

Oglesby says the perception that combat sports and MMA are “tough guy” activities is a myth. “I think it’s the absolute opposite,” he says. “It’s the geekier kids who come in here. I think it’s because it’s quite cerebral.”

Not bling

No doubt there are also many who are chasing the Las Vegas dream of expensive cars and champagne for breakfast, but Oglesby says the level of wealth enjoyed byMcGregor is "really rare" in MMA.

"You're going to be lucky to be driving a Toyota Starlet when you're done," he says. "What Conor McGregor has – the money, driving around California in a Rolls Royce – has happened to Conor McGregor and maybe five other people.

“Everybody else is lucky to make a living. They will pay their bills hopefully. There are some people fighting at the very top level that are struggling financially. I’d say that’s most people. It’s not bling.”

When it comes to the adults taking part in MMA, a broad cross section of society is involved. Most take it up in favour of other disciplines or are there as fans of the sport. Many are in their 30s.

“People often say to me, ‘God, it must attract a certain type of person’,” says Oglesby. “I don’t agree. We’ve got two doctors coming today, a forensic scientist, a couple of gardaí, and then we’ve got carpenters, people who work in bakeries, unemployed guys.”

The sport is mostly, but not entirely male. Ultimate Fighting Championship president Dana White insisted he would never permit women to fight, and it took 20 years for him to sanction a women's MMA bout. Several women train at Kyuzo Gym.

“They tend to be quite high achievers,” says Oglesby. “That’s already in them. It helps them train very hard and get better, but it also helps them overcome that social barrier that is there for women.

“There are already so many things in the way for them, that to go on and try to fight is a really massive thing. They fight the social battle as well as the fight in the cage,” he says.

One of the women is Jessica Ryan (27), a doctor at Beaumont Hospital, who is still trying to catch her breath after a 90 minute session on the mats.

“I always wanted to fight,” she says. “All my life I wanted to fight – but in a controlled environment.

“When I tell people I do MMA, there is normally surprise because I’m a woman. That’s the talking point. When I started here there was only one other girl to train with. That was tough because she wasn’t always there. But you just suck it up and get over it.

“We’re not even 10 per cent of the total population of fighters. Although the number of girls competing has increased massively, it was really tough at the start. Nobody ever gives me stick for it, except my dad. He doesn’t want me getting injured or anything.”

Ryan says she takes a huge amount away from MMA. “You can’t think about anything stressful,” she says. “You’re 100 per cent focused on this so it takes you away from normal life problems. It’s really good for mindfulness.

“The discipline is great. It can be really tough sometimes and really frustrating but something like martial arts teaches you about discipline, and that spills into other parts of your life and helps you develop as a person as well as everything else.”

But what about getting hit? “I don’t enjoy it,” she says. “I can remember in my first session, I got hit straight – square in the nose – and it was a big shock, but you get used to it.”

The sport has been heavily criticised for the level of violence involved. Though Oglesby says he can “certainly see a lot of the points” that have been made, he insists that it “looks a lot worse than it is”.

“Understandably there’s been a lot of negative press over the past couple of months, but even before that we’d been dealing with the same stuff, and I find that a little frustrating,” he says.

“I’ve heard it described as organised thuggery and all this kind of stuff. It’s not like that. People will say you’re teaching them violence. I absolutely don’t agree. Violence is something that happens out on the street.

“In boxing, if there’s blood, it tends to be quite minimal because there are larger padded gloves involved. When there are smaller gloves, there are more cuts, but equally there are less concussive blows than boxing.

“There tends to be more blood involved, but it’s because the cut is bleeding, the adrenaline is pumping, it’s sweaty and it goes everywhere. I know the sport is as safe as it can be, when it’s done properly. There are risks involved and I accept those risks.

“I don’t think anybody gets into the cage to fight who doesn’t know the risks and doesn’t accept them. I can see why people think it’s distasteful and it’s not what they want to see, but they don’t have to watch.”

Striking in the head

Carvalho’s death after the National Stadium fight left the entire MMA community shocked, he says. “There was a huge amount of sadness. That was the first reaction.

“I wasn’t at the show but a few of the guys were and I remember texting them on Sunday morning asking how the night went. They said something terrible had happened and I knew from the reaction that it was bad.

“We found out initially that Joao Carvalho was ill. There was a real sense of a pulling together. I’m not a religious man but you know that sense of just hoping against hope that he would be okay.

“We couldn’t believe it. It’s a strange one because, of course, it’s a sport that involves striking someone in the head.

“We all are aware of that. For the community as a whole, it was a horrible time.”

Much of the criticism that followed Carvalho’s death was unfair, he argues. “When you’re reading the newspaper and listening to the radio, it seemed like everybody had an opinion,” he says.

“There was some stuff I couldn’t disagree with. There were some counter arguments which I thought raised some really good points. Then there were other things written and you’re just shaking your head as you read them.

“Nobody wanted to speak to the press at the time. When somebody dies playing sport, it’s indefensible in the first instance. We needed to step back and see what went wrong and what happened.

“People were really taken aback and everybody did start to ask themselves questions about the sport. If you don’t ask yourself questions after something like that happens, there’s something wrong with you.”

Brain injury

So were fighters frightened by the death? “The thought of brain injury should scare everyone,” he says. “It should scare anybody involved in impact or collision sports or combat sports.

“People ask me on the street. You can’t hide from it. It is a sport that carries inherent danger, so why not deal with that and say it does carry that, in the same way rugby does. Let’s deal with it and talk about it.”

Carvalho’s death has accelerated the process of properly regulating the sport in the Republic, and a new national Governing body is on the way. Oglesby says there are going to be “massive changes” for amateurs.

“They’re going to be wearing shin pads and rash guards for one thing,” he says. “It’s a big sea change for MMA. I don’t think anyone envisaged the governing body would come in so quickly.

“It’s been in the works for a couple of years, but in the wake of Joao Carvalho, and the looking inward we’ve had to do over the last couple of months, this is the big push people are going for now.”

Nathan Kelly (19) from Finglas is an assistant at Aldi. He arrived early to do some extra training in preparation for representing the Republic at the IMMAF World Championships in Las Vegas. He says he is "buzzing"for the competition.

“When I raised MMA the first time, my ma said I couldn’t do it,” he says. “I think she was afraid I was going to get my teeth knocked in. But I am always in a controlled environment that is safe. Everyone looks out for each other.

“I love the training most of all but the fights are great too. I love the nerves. They drive you on. It’s deadly. I find it centres me a lot. When I was doing the Leaving Cert, it would always keep me at peace and stopped me from losing the head. It was a weight lifted.

“It gets rid of all your stress. It’s just a different environment altogether – real peaceful – it’s like my second life. A lot of people say it helps with depression as well, and even self-confidence issues. It gives you a bit of belief in yourself that you’re able to do something. I wasn’t very confident as a kid.”

Aidan Mulligan (26) from Blanchardstown works at St Patrick's Hospital. He became a fan of the sport in the late 1990s, and a fighter later: "Discipline is part and parcel of any martial art," he says.

“As with anything, you get out what you put in. That starts with being on time, being prepared, being willing to learn, not talking while someone is trying to teach you something, and it instils that in everyone.

“If you come down here when there’s a kids’ class on, you can see the discipline being instilled in them from an early age. Everybody is in line and here to learn. It’s not just a bunch of ill-disciplined youngsters. It’s inspiring.”